This study will evaluate the impact of an innovative integrated resource center for homeless and marginally housed drug users in San Francisco. Homeless and marginally housed drug users are at high risk for severe physical and mental health problems that are exacerbated because they are isolated, uninsured, and underserved. In light of the myriad of barriers to health care utilization, it is critical to the public health agenda that new and innovative interventions attempt to alleviate these barriers and their health consequences. It is equally critical that these innovative interventions undergo rigorous scientific evaluation. The past decade has seen the emergence of integrated resource centers serving drug users, which provide a variety of services under one roof. The Mission Neighborhood Resource Center (MNRC) is opening in May, 2002, and will integrate primary care, mental health care, substance abuse counseling, dental care, vocational training, housing advocacy, peer counseling, and basic personal services (including showers, lockers, voicemail, and storage). The aims of this study are (1) to assess whether MNRC participants are more likely than those in a comparison group to reduce their HIV risk behaviors, (2) to assess whether MNRC participants are more likely than those in a comparison group to access drug treatment, (3) to assess whether MNRC participants are more likely than those in a comparison group to report an increase in quality of physical and mental health, and (4) to assess whether MNRC participants are more likely than those in a comparison group to increase utilization of primary health care and reduce emergency room utilization. To accomplish these aims, six serial cohorts of 80 injection drug users (IDUs) and crack cocaine smokers (CCSs) will be recruited and followed for six months (n=480). Each cohort will consist of 40 MNRC intervention participants and 40 comparison subjects from a nearby neighborhood. We will investigate Specific Aims 1, 3 and 4 by examining if MNRC clients are more likely than those in the comparison group to report changes in HIV risk behavior, physical and mental health, and primary health care utilization. We will investigate Specific Aim 2 by comparing entry to drug treatment among MNRC clients to the comparison group. Qualitative interviews will also be conducted with clients (N=30) and staff (N=6), to gain a richer contextual understanding of the impacts of the MNRC.